This invention is particularly directed to improvements in photovoltaic devices and the like and particularly such devices which include at least one semiconductor layer which is a thin polycrystalline film of suitable large grain size. One such semiconductor material is zinc phosphide (Zn.sub.3 P.sub.2).
Zinc phosphide is an important semiconductor for photovoltaic solar cells. See, for example, a report by applicant et al, which was published in the Proceedings of the Fourteenth IEEE Photovoltaic Specialists Conference, pp. 641-646 (1980) and references cited therein. It possesses a steep optical absorption edge near 1.5 eV, where the maximum performance of terrestrially based cells is expected, has a sufficiently long (.about.10 .mu.m) minority carrier diffusion length to permit high current collection efficiency, and is comprised of elements which are abundant, thus permitting widespread use. To date, conversion efficiencies as high as 6.08% (total area, 100 mW/cm.sup.2 ELH simulation) have been reported for metal-semiconductor junctions. Cells of the heterojunction type and cells comprising solid solutions of zinc phosphide and cadmium phosphide are described in commonly assigned U.S. patent application Ser. No. 944,971, the details of which are incorporated herein by reference thereto. Cells of the homojunction type are described in said U.S. Pat. No. 4,342,879.
A particular problem with such semiconductor materials as zinc phosphide is in the difficulties encountered for selecting a proper substrate which meets the necessary criteria while being compatible with high temperature growth. For example, the relatively high coefficient of thermal expansion of zinc phosphide (1.4.times.10.sup.-5 /.degree.C.), compared to those of other semiconducting materials, such as silicon, which have a coefficient of thermal expansion of 4.5.times.10.sup.-6 /.degree.C., makes it difficult to bond zinc phosphide to conventional substrate materials as are commonly used in making thin film photovoltaic cells. This problem is particularly apparent in view of the temperatures to which these materials are exposed in the normally employed close spaced vapor transport (CSVT) method of forming zinc phosphide films.
Commonly assigned application Ser. No. 944,971 teaches a mica substrate with a layer of silver covered by a thin diffusion barrier of carbon as a substrate for forming zinc phosphide semiconductor layers of Schottky barrier and heterojunction type photovoltaic cells. The parent application teaches, inter alia, homojunction photovoltaic cells comprising zinc phosphide deposited on a multilayer substrate. Pasierb in U.S. Pat. No. 3,368,125 teaches use of a thin germanium layer in combination with a tin layer on a molybdenum substrate for depositing gallium arsenide films. The teaching of Pasierb has not been, to our knowledge, applied to semiconductor materials from Groups IIB and VA of the Periodic Table such as zinc phosphide. Furthermore, even if the teaching of Pasierb were operable with zinc phosphide and the like, the high cost of germanium and molybdenum would mitigate against their use in photovoltaic devices for large scale generation of electricity. A publication by Sberveglieri et al which appears in Thin Solid Films, Vol. 83, pp.L133-L136 (1981) teaches use of an evaporated layer of thallium in connection with a polished stainless steel substrate for deposition of zinc phosphide thin films. The teaching of Sberveglieri et al involves annealing for periods of up to eight hours. In view of the expense and toxicity of thallium and the long processing times inherent in this method, such substrates are unlikely to be useful for commercialization of zinc phosphide comprising thin film photovoltaic cells.
Photovoltaic cell devices as taught in the prior art can suffer from low yield and loss in conversion efficiency due to low fill factor. These problems are attributable to properties of the substrate such as high contact resistance, unwanted doping, incidence of pin holes, loss of adherence, extreme roughness and small grain size. Thus it would be advantageous to provide a substrate which overcomes these problems.